The Formation Mechanism of Damaging Surface Wind during the Squall Line in June 2009

A severe occlusion type of squall line is investigated by using observational data analysis and WRF model simulation, which occurred in Henan Province in China on June 3, 2009 and produced surface damaging high wind. The observational data analysis result shows that: The right branch of this system has some similar features to typical squall lines. Some features such as thunderstorm high, boundary outflow, cold pool, and wake depression are presented in the mature stage. By contrast, for the right branch there are not significant features such as stratiform cloud, thunderstorm high, and cold pool in the left branch of the squall line. The surface wind within the region of left branch is weaker than that in the right branch. High wind was mainly caused by the right branch of the system. High-resolution model outputs reveal that the formation of occlusion morphology of this system is caused by cold wind shear and warm wind shear near the ground. These shears lead to the existence of right branch with stratiform cloud and left branch without stratiform cloud, respectively. Furthermore, the formation mechanism of the surface high wind and the organization mode of the right branch of this system in the mature stage have been revealed: The downward motion of the convective region in the right branch is intense in developing and mature stage. Rear inflow in the middle troposphere may intensify such downward motion. Rear inflow is an important factor for surface high wind. There are three major air flows within the cross section of the right branch, including front-to-rear inflow in front of the squall line, rear-to-front inflow in the middle layer, and inflow in the lower layer which changes to outflow in the high layer.